utvideo: fix gradient prediction for interlaced video

[ami_stuff]

decodes bit-exact with vfw codec

​https://files.fm/u/8ebkzaug

C:\>ffmpeg -i utv_rgb_int_gradient_8.avi out.avi
ffmpeg version 3.3.git Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-asm --disable-pthreads --enable-gpl --disable-ffprobe

  libavutil      55. 61.100 / 55. 61.100
  libavcodec     57. 93.100 / 57. 93.100
  libavformat    57. 72.101 / 57. 72.101
  libavdevice    57.  7.100 / 57.  7.100
  libavfilter     6. 85.100 /  6. 85.100
  libswscale      4.  7.101 /  4.  7.101
  libswresample   2.  8.100 /  2.  8.100
  libpostproc    54.  6.100 / 54.  6.100
[utvideo @ 0222cd78] Warning: not compiled with thread support, using thread emu
lation
Input #0, avi, from 'utv_rgb_int_gradient_8.avi':
  Duration: 00:00:03.20, start: 0.000000, bitrate: 10899 kb/s
    Stream #0:0: Video: utvideo (ULRG / 0x47524C55), rgb24, 500x500, 11227 kb/s,
 10 fps, 10 tbr, 10 tbn, 10 tbc
[utvideo @ 0233aba8] Warning: not compiled with thread support, using thread emu
lation
Stream mapping:
  Stream #0:0 -> #0:0 (utvideo (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
[mpeg4 @ 0233c1f0] Warning: not compiled with thread support, using thread emula
tion
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf57.72.101
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 500x500, q=2-31, 200
 kb/s, 10 fps, 10 tbn, 10 tbc
    Metadata:
      encoder         : Lavc57.93.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=   32 fps=0.0 q=31.0 Lsize=     317kB time=00:00:03.20 bitrate= 812.7kbits
/s speed=6.61x
video:311kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: 2.030940%

[Elon Musk]

Fixed in 49255370044cf4a58c81a88cb8206aee62086346.